In recent years, it has been common for electronic devices, such as mobile phones, laptop computers, electronic notebooks, digital cameras, and gaming machines, to have audio functions. These devices are required to have a waterproof structure. A housing of an electronic device with audio functions is usually provided with openings located at positions corresponding to a sound emitter and a sound receiver such as a speaker, a microphone, and a buzzer. Since sound needs to be transmitted through these openings, which makes it difficult to form a waterproof structure while ensuring the audio functions. It has been attempted to ensure both sound transmission through an opening of a housing and waterproofness of the opening by covering the opening with a waterproof sound-transmitting membrane. The waterproof sound-transmitting membrane is a thin film made of a material that is less likely to block transmission of sound. Therefore, this waterproof sound-transmitting membrane placed over the opening prevents entry of water into the housing while ensuring good transmission of sound. A gas-permeable membrane that allows passage of gasses but prevents entry of water is suitable for use as such a waterproof sound-transmitting membrane, and more specifically, a gas-permeable membrane having a porous polytetrafluoroethylene (PTFE) membrane is suitable (see Patent Literature 1).
A gas-permeable membrane having a porous PTFE membrane can be used not only as a waterproof sound-transmitting membrane but also as a waterproof gas-permeable membrane or a dustproof gas-permeable membrane that allows passage of gases but prevents entry of water and/or dust. However, a gas-permeable membrane may be exposed to sebum, a surfactant, oil, or the like, depending on the environment in which the membrane is used. Even if a highly water-repellent porous PTFE membrane is used as a gas-permeable membrane, entry of a liquid with a low surface tension cannot be fully prevented. Therefore, the gas-permeable membrane is subjected to liquid-repellent treatment using a treatment agent containing a fluorine-containing polymer capable of imparting oil repellency.
It is well known that a fluorine-containing polymer having a linear perfluoroalkyl group (hereinafter, a “linear perfluoroalkyl group” may be referred to as an “Rf group”) having 8 or more carbon atoms is suitable for imparting liquid repellency. Rf groups having 8 or more carbon atoms have significantly higher crystallinity than Rf groups having a smaller number of (for example, 6 or less) carbon atoms. This high crystallinity is considered to contribute to the development of excellent liquid repellency. It is also known that, due to its high crystallinity, a treatment agent having an Rf group having 8 or more carbon atoms has a large receding contact angle, (which is one of the dynamic contact angles and the other of which is an advancing contact angle). The receding contact angle increases with increasing crystallinity, and sharply increases as the number of carbon atoms increases from 6 to 8. For these reasons, it is common practice to use a treatment agent containing a fluorine-containing polymer having an Rf group having 8 or more carbon atoms so as to impart liquid repellency to a gas-permeable membrane.